The present invention relates to a display control device, and in particular to a liquid crystal display control device for a portable equipment or the like.
FIG. 9 shows a display control device of a command control type. In FIG. 9, reference numeral 1 denotes an image data writing means including a CPU provided with an address bus, a data bus, and control lines. Reference numeral 2 denotes a graphics memory storing write data from the image data writing means 1. Reference numeral 3 denotes a data transfer means for reading, from the graphics memory 2, image data having been written by the image data writing means 1, and transferring the data to a display means 4. The display means 4 displays images, and includes a memory 5, a liquid crystal driver circuit 6 and a liquid crystal panel 7. The memory 5 stores image data for one screen of N dots (arranged in the horizontal direction)×M lines (arranged in the vertical direction) (N and M being positive integers) transferred from the data transfer means 3. The liquid crystal driver circuit 6 reads the data from the memory 5 responsive to clocks in synchronism with a display frequency, and drives the liquid crystal panel 7. The liquid crystal panel 7 is driven by the liquid crystal driver circuit 6 to display the image data.
In the display control device described above, as shown in FIG. 10, the image data for one screen is written from the image data writing means 1 such as a CPU or the like in the graphics memory 2. In this instance, not the entire screen of data is written, but only such part (pixels) of the screen of data that needs to be updated is rewritten. The data written represents images, characters, or the like. The image data in the graphics memory 2 is read by the data transfer means 3 sequentially from the address 0 to address N×(M-1). The data read is output to the display means 4, after addition of a command setting the horizontal address and the vertical address of the write region, e.g., a command as shown in FIG. 11. The display means 4 decodes the input command, and writes one screen of data in the region of from address 0 to address N×(M-1) in the memory 5. The data for one screen having been written in the memory 5 is read by the liquid crystal driver circuit 6 responsive to clocks in synchronism with the frame frequency of the liquid crystal display by the liquid crystal panel 7, and liquid crystal driving waveforms are thereby generated, and images are displayed by the liquid crystal panel 7.
Since the conventional display control device is configured as described above, when the data is transferred to the memory 5, one screen of data is transferred every time (every frame period). As a result, even when the data written from the image data writing means 1 to the graphics memory 2 is updated with regard to a small area of the screen, the transfer means 3 transfers the entire screen of data from the graphics memory 2 to the memory 5. The amount of power consumption of the circuit operating for the data transfer is the same as that required for rewriting the entire screen, so that the efficiency is low, and the useless power consumption occurs.